The invention relates to ceramic-metal compositions or cermets, especially of boron carbide and copper. More particularly the invention relates to densified cermets of B.sub.4 C and Cu that have selected desirable characteristics that are derived from having a predominate ceramic component.
The potential of ceramic-metal compositions that include a unique combination of properties, such as the hardness of a ceramic material combined with the ductility of a metal, has long been of interest. It has, however, proved difficult to achieve cermet compositions of many desirable materials that are fully densified and have a particularly desired range of properties. The difficulties arise form the conflicting physical and chemical nature of the starting materials.
Major areas of difficulties in processing of ceramic-metal composites are associated with:
(i) chemical reactions of the starting materials such as oxidation and/or other reactions leading to metal depletion and formation of undesirable phases between ceramic and metal; and PA1 (ii) non-wetting behavior of the metal with respect to the ceramic component.
These difficulties lead to cermet products that have a higher porosity than desired and lack desired ceramic-metal phases that would, if present, impart needed properties to the cermet.
Where the metal chosen for the cermet composition is, for example, non-reactive and non-wetting, such as copper in a boron carbide ceramic phase, there is serious difficulty in achieving a fully dense cermet that is high in ceramic content. The difficulties in manufacturing B.sub.4 C/Cu cermets are associated with the non-wetting characteristics of Cu for B.sub.4 C. If, in making a B.sub.4 C/Cu cermet, the reaction mixture is heated above the Cu metal melting temperature, a conventional processing step in densifying composite compositions, Cu forms spheres that separate from the mixture of ceramic and metal particles. In addition, where a high ceramic content is desired and the particle size of the ceramic component is reduced toward that object, the finer the ceramic particles and the greater the ceramic content of the mixture, the more metal migrates out of the bulk mixture towards the surface of a porous compact of the ceramic. Obtaining high ceramic content cermets of high density of materials such as boron carbide and copper has heretofore proved substantially impossible. Heating to even high temperatures does not improve densification because the wetting of B.sub.4 C by Cu is not a function of temperature, in contrast to an aluminum-boron carbide system, for example, wherein the metal wets the ceramic and can, thus, infiltrate into a porous ceramic compact system. When the non-wetting metal melts, the system tends to separate and becomes uncontrollable with respect to densification.
Therefore, all prior work with the B.sub.4 C/Cu system has been conducted below the melting temperature of Cu in order to densify cermets. Below the metal melting point, a metal component must be physically deformed and forced into the small channels between the ceramic grains of a ceramic body. The smaller the ceramic grains, the smaller are the channels between the grains and, as a result, the more difficult it is to sufficiently deform the metal to achieve a dense cermet structure. As a consequence, the maximum amount of ceramic and minimum particle size that can be employed in making B.sub.4 C-Cu cermets has, heretofore, been limited. Keil, et al., in "A Feasibility Study for Fabrication of Cu-B.sub.4 C Sheet", LA-3570-MS, Los Alamos Scientific Laboratories, Los Alamos (1965), describes producing 2.5 mm thick sheets of 50 volume percent B.sub.4 C/Cu by means of a process combining vacuum hot pressing and hot rolling.
Smugersky, et al., in "Development of B.sub.4 C/Cu Cermets", Sandia Laboratories 78-2317, TTC/0017 (1978) utilizes a hot isostatic pressing technique to make B.sub.4 C/Cu cermets, pressing at 103 MPa and 500.degree. C., for three hours. Smugersky's experiments indicated that full density could be obtained only if the total B.sub.4 C ceramic component content is below 60 volume percent. Densities of 97.5 percent and 88.2 percent are obtained with B.sub.4 C contents of 70 and 80 volume percent, respectfully, but at greatly reduced strength. In these experiments a coarse B.sub.4 C powder, greater than 44 micrometers average particle size, was required and attempts to use finer B.sub.4 C powder led to cermets of high porosity. The bending strength produced in the Sandia experiments, in terms of transverse rupture strength, were 40 and 56 percent lower, for the 70 and 80 volume percent material, respectively, than the 87 MPa achieved for the 60 percent cermet. According to the work done in Sandia, the maximum content of B.sub.4 C in B.sub.4 C/Cu cermets has been limited to less than about 60 volume percent with a minimum B.sub.4 C grain size of about 44 micrometers.
For ceramic-metal systems that are non-reactive and/or non-wetting, such as boron carbide and copper, it would be desirable to produce ceramic-metal compositions having a greater than 60 volume percent ceramic content, in order to provide densified cermet compositions that include ceramic characteristics that are advantageous for many uses, as well as the characteristics of the metal. What is needed are cermet compositions having a greater flexibility in composition, preferably including an increased ceramic content, such that a desirable quality of an individual ceramic component may be emphasized.